Waldminghaus:Back Door/glycerol culture
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To preserve strains for many years and for example store them in a strain collection you should mix 1ml of liquid culture with 600μl of a sterile 50% glycerol solution (end concentration: 18.75%) in a cryo tube and store at -20 or -80°C. A log phase culture is optimal but an over night culture also works (if you have a tricky mutant strain you should use a log phase culture).
The described method works for E. coli, V. cholerae, A. tumefaciens, yeast and probably many others.